1. Field of the Invention.
The present invention relates to the field of optics and in particular to a reflective stage overhead projector utilizing an asymmetrical reflective supporting stage.
2. Description of the Prior Art.
Overhead projectors are in widespread use because they provide an economical and efficient method of displaying and presenting information to a large number of spectators. Overhead projectors generally have the following basic components: a base containing an asymmetrical reflective stage such as a Fresnel lens which provides support for the transparent material to be displayed; a lamp and lens combination positioned above the stage to illuminate the reflective stage; and a mirror and lens assembly through which rays of light radiating from the reflective stage are focused onto a screen.
The above components work in combination to display information contained on the transparent material to the screen. The lamp and lens combination is designed to provide an intense illumination to the reflective stage. Light radiating from the reflective stage is directed toward the mirror and lens assembly which focuses the image onto the screen.
The reflective stage is composed of two surfaces: a first planar surface and a second reflective surface which is typically formed utilizing a portion of a Fresnel lens. The type of Fresnel lens typically used in overhead projectors has a small planar center with a plurality of circular concentrically stepped setbacks which reflect illuminating light to a target point above the lens. The location of the target point will change corresponding to changes in position of the lamp and lens combination. The lamp and lens combination and the reflective surface are placed in a fixed position relative to one another such that the target point of the Fresnel lens is located at the mirror and lens assembly. Light radiating from the reflective surface travels through the transparent material and converges at the target point where the mirror and lens assembly focus the light onto the screen.
Overhead projectors utilizing the above components are disclosed, for example, in the following references:
______________________________________ Country Applicant Reference No. ______________________________________ U.S.A Appeldorn 3,293,982 U.S.A. Hubner 3,486,817 U.S.A. Mueller 4,206,984 Germany Demolux GmbH & Co. KG 34 10 024 A1 Germany Demolux GmbH & Co. KG 35 12 466 A1 ______________________________________
Appeldorn, Hubner and Mueller each show an overhead projector with its mirror and lens assembly located between a light source and a screen on which an image is to be projected. The Demolux references reverse the positioning of the mirror and lens assembly and the light source, such that the light source is located between the mirror and lens assembly and the screen. The light source is so positioned in Demolux reference 34 10 024 A1 to accommodate a slide projector and in Demolux 35 12 466 A1 to accommodate an auxiliary support surface.
The optical configurations utilized in the prior art have limitations which cause several problems to both the presenter (user) who views the stage area as well as to the audience which views the illumination of the projected image on the screen. One limitation is a "bright-spot" of light projected upon the screen of higher intensity than the surrounding illumination. Another limitation, termed "hot-spot," is a reflection of the lamp off of the reflective surface which presents a glare to a user of the projector.
A "bright-spot" refers to an area or spot of light on the screen of higher intensity than the surrounding illumination. As discussed above, a Fresnel lens has, on one side thereof, circular concentric setbacks which reflect illumination to a target point above the lens. Proper reflection to this target point requires that a small circular center area of the Fresnel lens be planar. The planar center area of the Fresnel lens acts as a mirror and reflects not only the illuminating light from the lamp but an image of the lamp itself. The mirror and lens assembly, viewing the lamp off of the planar surfaces of the transparent material, the first planar surface of the reflective stage and the planar center area of the Fresnel lens, forms a real image of the lamp in the space between the mirror and lens assembly and the screen. The light from the real image is projected on toward the screen where it forms a spurious defocused spot of light of higher intensity than the surrounding illumination. The spot of light is annoying to a viewing audience and causes a loss of image contrast in a region where the spot of light strikes the screen.
The "hot-spot" limitation, contrasted with the "bright-spot" problem discussed above, is not formed because of any particular characteristic of the Fresnel lens. Rather, it is a specular reflection of the lamp image off of the planar surfaces of the transparent material and the first planar surface of the reflective stage. Because of this specular reflection, a user looking at the reflective stage will see a virtual image of the lamp reflected off of these planar surfaces. This lamp image creates a glare which is annoying to the user viewing the reflective stage, causing eye strain and making presentation of material more difficult.